This invention relates to a liquid-crystal matrix display having
two plane-parallel carrier plates which, together with a frame, form a cell,
a nematic liquid-crystal mixture contained in the cell and having positive dielectric anisotropy and high specific resistance.
Such liquid-crystal matrix displays (LCM displays) are known. As nonlinear components for the individual switching of the individual image points, use may be made, for example, of active components (e.g., transistors). In that case, reference is made to an "active matrix", a distinction being made between two types:
1. MOS (Metal Oxide Semiconductor) transistors on a silicon wafer as substrate,
2. Thin-film transistors (TFT) on a glass plate as substrate.
In the case of type 1, the dynamic scattering or the guest-host effect is normally used as the electro-optical effect. The use of monocrystalline silicon as substrate material limits the size of the display since even the modular combination of different subdisplays leads to problems at the joints.
In the case of the more promising type 2, which is preferred, the TN effect is normally used as the electro-optical effect. A distinction is made between two technologies: TFTs made from compound semiconductors such as, for example, CdSe or TFTs based on polycrystalline or amorphous silicon. The latter technology is being worked on worldwide with great intensity.
The TFT matrix is applied to the inside of a glass plate of the display, while the other glass plate carries the transparent counterelectrode on the inside. Compared with the size of the image point electrode, the TFT is very small and virtually does not disturb the image. This technology can also be extended for fully color-capable image representations in which a mosaic of red, green and blue filters are arranged in a manner such that a switchable image element is situated opposite each filter element.
The TFT displays normally work as TN cells with crossed polarizers in transmission and are illuminated from behind.
Here the term LCM displays encompasses any matrix display having integrated nonlinear components, i.e. in addition to the active matrix, also displays having passive components such as varistors or diodes (MIM=metal-insulator metal).
Such LCM displays are suitable, in particular, for TV applications (for example pocket television sets) or for displays with high information content in motor vehicle or aircraft construction. In addition to problems relating to the angular dependence of the contrast and the switching times, difficulties arise in the case of LCM displays due to the insufficiently high specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, Sept. 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, Sept. 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. As the resistance decreases, the contrast of an LCM display deteriorates. Since the specific resistance of the liquid-crystal mixture in general decreases over the lifetime of an LCM display due to the interaction with the inner surfaces of the display, a high (initial) resistance is very important in order to obtain acceptable service lives.
There is consequently still a great need for LCM displays with very high specific resistance accompanied at the same time by large working temperature range, short switching times and low threshold voltage.